vbi teletext is a format specification that allows for including text (characters) and a few basic images as metadata between video frames of a broadcast.

libzvbi_teletextdec can decode that, and output in two formats, either "image" output or "text" output. Default is image. Text output must be selected in order for it to be converted to other text formats, like, in this instance, srt. See ​here for more options.

You can also convert to .ssa format, etc. Here is how to list all the subtitle options available: